1. Field of the Invention
The present invention relates to a technique for correcting image registration in an image forming apparatus employing an electrostatic recording system or an electro-photographic recording system.
2. Description of the Related Art
Recently users have demanded high speeds and high image quality as well as conformity with various kinds of recording sheets (recording media) for image forming apparatuses. This provides particular challenges for color image forming apparatus, where multi-color images are sequentially overlapped on a transfer member so as to form color images.
As a means for increasing speed, a tandem type of recording system is provided in which a plurality of image forming sections for each color are arranged in a row. The speed of the system can be further increased by reducing the gap between pages or recording sheets, i.e., between images continuously formed during continuous printing.
In order to improve image quality, in both the monochromatic and color image forming apparatuses, the displacement relative to a recording sheet is reduced by improving accuracies in position for forming images on the recording sheet. In a color image forming apparatus, high image quality is also achieved by improving accuracies in position when multi-color images are overlapped on one transfer sheet so as to reduce color misalignment. Furthermore, in the color image forming apparatus, high image quality is achieved by reducing density misalignment between desirable images and images formed by the image forming apparatus in practice. For correcting the position misalignment, the color misalignment, and the density misalignment, the image forming job can be stopped (interrupted) so as to form a mark train on a transfer member for detecting the misalignment based on the detected result. After adjustment, the image forming job can be resumed.
However, since the job has been interrupted for correction (adjustment) in this method, the productivity is reduced. In addition, a problem arises in that image quality is different before and after the adjustment is made during the same job.
It has been suggested in Japanese Patent Laid-Open No. 2005-292760 that adjusting color misalignment without interruption can be achieved by performing the correction in the gaps (intervals) between images continuously formed during continuous printing.
When images are formed on both sides (front and rear sides) of one recording sheet, the adjustment of size and position of the images on both sides (double-side registration) is demanded. When toner images formed on the recording sheet are fixed by heating, since part of the moisture contained in the recording sheet is evaporated due to the fixing heat, the recording sheet can contract after the fixing so as to generate dimensional change. This dimensional change is different for each type of recording sheet.
In a two-sided printing mode, after images are fixed on the front side (first side), images are formed on the rear side (second side) in a state that the recording sheet has contracted by about 0.1 to 0.5%. Hence, when the recording sheet by absorbs water after the images are formed on the rear side, the recording sheet and images expand, so that the image size of the front side is different from that of the rear side, resulting in poor front to back image registration. An image forming apparatus is proposed in Japanese Patent Laid-Open No. 2002-014507 for improving image quality by solving the problems during the two-sided printing described above, by changing the rotational speed of a polarizer (rotary polygon mirror), which is scanning means for writing images, to align the double-side image dimensions during two-sided printing.
As described above, for speeding up and improving image quality, it is effective to align images in the space between continuously formed images. Also, in improving image quality during two-sided printing, a technique changing the rotational speed of the polarizer (rotary polygon mirror) is effective for the image dimensional alignment.
Accordingly, for speeding up and improving image quality, it may be effective to change the rotating speed of the polarizer (rotary polygon mirror) in conformity with the dimension of the continuously formed images (i.e., changing the image dimension) as well as to align the image in the space between continuously formed images.
A sequence of reference marks for detecting the misalignment needs to be formed at a known reference image size as disclosed in Japanese Patent Laid-Open No. 2005-292760. However, the size of the reference marks for detecting the misalignment for double sided images must be changed between the imaging of the first side and the second side. Therefore, the rotational speed of the polarizer must be changed so that the image dimension becomes 100%. That is, the rotational speed of the polarizer is changed for writing so that the image dimension at a first time (front side) becomes 102%; after writing, the rotational speed of the polarizer is changed so that the image dimension becomes 100% so as to form the reference marks on the other side for detecting the misalignment. Furthermore, the rotational speed of the polarizer is changed for writing so that the image dimension at a second time (rear side) becomes 98%; after writing, the rotational speed of the polarizer is changed so that the image dimension becomes 100% so as to form the reference marks for detecting the misalignment. These operations are to be repeated.
During changing the rotational speed of the polarizer, images cannot be formed. Also, the rotational speed of the polarizer is not stabilized until a predetermined time lapses after changing it, so that with increasing number of times of changing the rotational speed, the space between continuously formed images is increased, reducing the productivity of printing.
Accordingly, the number of times of changing the rotational speed of the polarizer is desirable to be as small as possible. That is, it is desirable that the reference marks for detecting the misalignment be formed without changing the rotational speed of the polarizer.
The present invention addresses these problems and others which will be understood from the following description.